It is proposed that the low fill factor in aSi solar cells is due to the field-dependent photocurrent collection. The photocurrent dependence on the width of the depletion region has been theoretically analyzed as a function of the hole diffusion length Lp. The analysis showed that Lp is not larger than 0.05 µm, and also that the depletion width at zero bias under AMI illumination is in the range 0.15-0.30 µm.